Beer’s Law Lab
Visible Radiation
• Recall, visible light represents a small region of the electromagnetic spectrum:
Beer’s Law Introduction
• Why are some solutions colored?
• Also, how does the observed color intensity depend on solution concentration?
• We will explore these questions by studying Beer’s Law?
Beer’s Law Intro. (cont.)
1
Incident “white” light
Transmitted light
Sample
• Sample looks red because only wavelengths corresponding to red pass through the
sample and are detected by your eye.
• This implies that color is due to the absorption of certain wavelengths by the sample.
I
o
Incident light at a specific wavelength (I )
o
• We can define the percent transmitted intensity(%T) as follows:
I
%T  100% o
I
I
2
Fraction of incident light
that passes through sample (I)
Sample
Wavelength (nm)
• Green and yellow wavelengths have high transmittance.
% Transmittance
Beer’s Law Intro. (cont.)
• We perform an experiment and measure %T at a given wavelength versus concentration
and find the following:
Concentration (M)
• Not linear with concentration.
% Transmittance
Beer’s Law Intro. (cont.)
• If we define the Absorbance (A) as A = -log(I/I ), we find:
o
Concentration (M)
• A is linearly proportional to concentration ( c ).
Absorbance
Absorption Spectrum of Chlorophyll a
Wavelength (nm)
• Maximum absorbance at ~430 nm and ~670 nm.
Beer’s Law Intro. (cont.)
• We can also measure absorbance versus sample thickness, or path length (b) at a
constant concentration :
• A is proportional to pathlength ( b ).
Absorbance
Beer’s Law Intro. (cont.)
3
• The relationship between absorbance (A), concentration ( c ), and pathlength is
expressed by Beer’s Law:
A  bc A εbc
A: absorbance (unitless) c: concentration (M)
b: pathlength (cm)
ε: the molar extinction coefficient (M
-1
-1
cm )
• In this lab, you will verify the relationship between A and c as described by Beer’s Law.
In Lab
• You will measure the absorbance of a standard solution.
• You will measure a blank solution (I ), and the absorbance of six standard solutions. If
o
Beer’s Law is correct, a plot of A versus c should be a straight line
A εbc
Slope will equal εb, with knowledge of b, you can determine ε.
• You will measure the absorbance of a standard solution
• You will measure a blank solution (I ), and the absorbance of six standard solutions. If
o
Beer’s Law is correct, a plot of A versus c should yield a straight line:
A εbc
• Slope will equal εx b, with knowledge of b, you can determine
ε for Ferroin at your
detection wavelength. You will use this value in the second half of the lab.
Dissolve in water, and take 5 ml of sample and dilute. Measure absorbance of solution.
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